1. Field of the Invention
The present invention relates to a surface discharge element and a method of making the same, and more particularly to a surface discharge element appropriate for use in an ozonizer using surface discharge for producing ozone or in an ionizer producing low-temperature plasma, and a method of making such a surface discharge element. The term,"surface discharge" is used as equivalent to two-dimensional silent discharge or corona discharge.
2. Prior Arts
A surface discharge element comprises a dielectric substrate having a relatively small, discharge electrode on one surface and a relatively large, inductive electrode on the other surface of the substrate. Application of a high AC voltage between the opposite electrodes causes low-temperature plasma to appear around the discharge electrode, and then an inductive current (discharge current) flows through the dielctric substrate between the opposite electrodes. In case of application of the surface discharge element to the ozonizer a high frequency (50 Hz to 20 KHz), high voltage (3.5 KVpp to 10 KVpp) is applied to the surface discharge element to produce oxygen ions, which are allowed to bond surrounding oxygen. Thus, ozone can be effectively produced. The discharge electrode of the surface discharge element is made of tungsten (W), titanium dioxide (TiO.sub.2), titanium nitride (TiN) and other materials. In making a discharge electrode by using for instance, tungsten an electrode pattern is printed on a ceramic substrate with tungsten, and then, the so tungsten-printed substrate is subjected to firing at 1,300.degree. C. in a hydrogen furnace. In case that a discharge electrode is made by using titanium dioxide or titanium nitride the substrate is plasma-spray coated with such materials to form a discharge electrode thereon.
The firing in the hydrogen atmosphere or the plasma spray coating, however, requires an extra equipment which is large in size, and very expensive. Also, disadvantageously such processes are not appropriate for mass production. As for the plasma spray coating the coating is liable to be peeled off from the alumina substrate. In an attempt to solve such problems a thick film paste including conductive powder and glass powder as main constituents, which paste is well known as being used in making printed circuits and chip resistors, is applied onto a ceramic substrate by printing, and the so printed ceramic substrate is subjected to firing process. The surface discharge electrode thus produced is liable to be easily broken during electric discharge, and the life of the surface discharge electrode cannot be significantly extended even if the electrode is covered with protective glass coating.
In view of the above what is aimed at by the present invention is to solve the problems of: the discharge electrode being fragile; the life of the discharge electrode being relatively short; and the discharge electrode being unable to be produced with ease.
From the angle of facilitating the making of such electrodes it is most advantageous that a thick film conductor paste is applied and fired onto an insulating substrate, but the so made electrode is easily broken during discharge. The inventor found that this fragility is attributable to presence of lead in the paste; lead is introduced to lower the firing temperature, and then lead contents are sputtered by electric discharges to leave the discharge electrode. More specifically the discharge electrode is composed of a thick film conductor including conductive powder and glass, and the glass contains lead in the form of PbO, Pb.sub.3 O.sub.4 and the like. The lead contents are sputtered and removed from the conductor by electric discharges, thereby reducing the strength of the discharge electrode.
One object of the present invention is to provide an improved surface discharge element whose discharge electrode is physically resistive to erosion by electric discharge, is long-lived for use, and is appropriate for mass production. Another object of the present invention is to provide a method of making such an improved surface discharge element.
To attain these objects a surface discharge element according to the present invention comprises a dielectric substrate having a discharge electrode on one surface and an inductive electrode on the other surface of the substrate, at least said discharge electrode being of a thick film conductor including conductive powder and lead-free glass as main constituents. Each of the discharge electrode and inductive electrode may have an insulating protective layer of lead-free glass thereon. Also, the insulating protective layer may contain an oxide filler to increase strength of the protective layer. Advantageously noble metal powder such as Au, Ag, Pd or Pt and their alloys, and powder of ruthenium oxides or other ruthenates may be used as conductive powder. Base metals such as Cu or Ni or their alloys may be used, too.
Prepared is a paste which contains as main constituents, powder of conductive material described above and lead-free glasses such as SiO.sub.2 --B.sub.2 O.sub.3 --ZnO glass, SiO.sub.2 --B.sub.2 O.sub.3 --ZnO--Al.sub.2 O.sub.3 glass, SiO.sub.2 --B.sub.2 O.sub.3 --ZnO-alkaline earth metal oxide glass, SiO.sub.2 --B.sub.2 O.sub.3 --ZnO--Al.sub.2 O.sub.3 -alkaline earth metal oxide glass, B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 -alkaline earth metal oxide glass, SiO.sub.2 --ZnO--Al.sub.2 O.sub.3 -alkaline earth metal oxide glass, and the paste thus prepared is applied onto one side of a dielectric substrate in the form of discharge electrode, and onto the other side of the dielectric substrate in the form of inductive electrode. The so applied electrode patterns are fired to provide a surface discharge element.
In an attempt to control the thermal expansion coefficient and other physical characteristics of the glass, alumina, zirconia, zircon, silica, cordierite, forsterite, mullite and other oxide filler along with coloring agents may be added to the glass paste. The protective layer has the effect of preventing the underlying electrode from being oxidized, and it is made of lead-free glass; glass containing lead is liable to lose its strength by allowing lead constituent to leave by discharge sputtering. Preferably the lead-free glass paste disclosed in Japanese Patent Application No.8-53,587 can be used in forming such protective layer.